The goal of operational Business Intelligence (BI) is to help organizations improve the efficiency of their business by providing information that can be used to make better decisions, aligning day-to-day operations with strategic goals. The creation and querying of customized virtual database schemas over a set of distributed and heterogeneous data sources may be implemented as part of this process. Thus, in many business entities, data is stored in multiple systems, each having different formats and access conventions. Unfortunately, the non-uniform distribution of data across disparate sources may also serve to frustrate business users seeking a more unified view of the data throughout their organization.